1. Field of the Invention
This invention relates generally to protection for a power converter. More particularly, the invention relates to protecting switching components (e.g., MOSFETs (metal oxide semiconductor field effect transistors) or IGBTs (insulated-gate bipolar transistors)) of a power converter.
2. Brief Description of Prior Art
One particular use of a DC/AC (direct current/alternating current) power converter, which is susceptible to a below resonant condition, is in a current generator to drive current for plasma generation in a plasma chamber processing thin films. The resonance in this environment is the pole resonance of a transfer function of a circuit made up of an output transformer of the power converter, a capacitor in series with the primary of the output transformer and the load impedance driven by the output transformer. The resonant frequency is determined by the load impedance, the leakage inductance of the transformer, and the capacitance of the capacitor.
In the DC/AC power converter two MOSFETs are connected in series. The primary of an output transformer is connected at a common node between the two MOSFETs so the pair of MOSFETs can push or pull current through the primary of the transformer during alternate ON/OFF cycles. However, during a below resonant frequency condition a MOSFET may be gated ON while the voltage across the MOSFET is high. Also, the paired MOSFET will at the same time be conducting current through its intrinsic diode. When the MOSFET biased with a high voltage turns on, it will cause the intrinsic diode in the paired MOSFET to snap off. This is referred to as a hard-driven condition for the paired MOSFET. Such a hard-driven condition can cause a MOSFET to fail over time.
In the past to prevent both MOSFETs from being hard-driven, four diodes have been added to the power converter. For each MOSFET there is an additional diode in parallel with the intrinsic diode of the MOSFET. This additional diode carries the reverse current that would otherwise be carried by the intrinsic diode of the MOSFET. Also for each MOSFET, a Schottky diode is added at the source connection to limit the reverse current through the intrinsic diode of the MOSFET. This solution to the hard-driven problem is expensive. In high power environments the high-current diodes added to protect the MOSFETs are expensive. Also, the additional high-current conductive paths are expensive because of the amount of precious metal used in the paths.